The present invention relates to the preparation of titanium disulfide and more particularly to the production of stoichiometric titanium sulfide having a high degree of crystalline perfection and a controlled particle size by directly reacting metallic titanium and elemental sulfur.
Recent work has shown that the dichalcogenides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, technetium, rhenium, platinum, germanium, tin and lead can be intercalated by Group Ia elements, Group Ib elements, Group IIa elements, Group IIb elements, ammonia (or substituted ammonium compounds), aluminum, gallium, indium and thallium to produce a new class of materials that have very interesting properties that make them useful as lubricants, X-ray diffraction grating crystals, super-conductors and thermoelectric elements. A potentially important commercial use for some of these materials is as active cathode materials in secondary batteries.
Titanium disulfide, in particular, has many properties that make it highly attractive as an active cathode material in secondary batteries. The one property makes titanium disulfide particularly useful as an active cathode material is that certain ions and molecules, such as lithium and ammonia, display high mobilities through titanium disulfide. These high mobilities allow titanium disulfide to be rapidly intercalated and provide high current densities when titanium disulfide is used as an active cathode material in an electrochemical cell. The high mobilities of ions and molecules in titanium disulfide, however, are not inherent properties but are dependent upon the composition of the titanium disulfide as well as its crystalline perfection. Titanium disulfide has a cadmium iodide structure with one layer per unit cell and with the sulfur being octahedrally coordinated about the titanium. Separate dichalcogenide layers are bound by van der Waals forces. Crystal imperfections due to growth conditions or to high metal concentrations result in interstitial titanium between the sulfur layers. Interstitial titanium drastically lowers the mobilities of ions and molecules and renders the titanium disulfide less desirable as a cathode material. It is thus highly advantageous to control the preparation of titanium disulfide to insure that the compound is stoichiometric and that crystal imperfections are minimized.
When transition metal dichalcogenides are used as active cathode materials in secondary batteries, the dichalcogenide is in the charged state when it is unintercalated. As the battery is discharged, the dichalcogenide is intercalated. The rate of intercalation of the dichalcogenide is a limiting factor of the current density that a battery can develop over prolonged periods without significant polarization occurring. Premature high polarization can prevent full utilization of the battery's theoretical capacity. Intercalation of the dichalcogenides is a diffusion process parallel to the basal planes of the hexagonal dichalcogenides. Large basal planes present greater diffusion distances which in turn results in lower battery discharge rates. When the transition metal dichalcogenides are to be used as active cathode materials, it is therefore highly advantageous that they be prepared in a manner that insures the production of crystallites with small aspect ratios, i.e., the ratio of the basal plane axis to the vertical axis. It should be noted that the lubricating properties of the dichalcogenides effectively eliminates comminution as a method for controlling particle size.
Titanium disulfide has been prepared by at least three methods. In one method titanium trisulfide is heated to a temperature of about 600.degree. C. to disproportionate the trisulfide to titanium disulfide and sulfur. High temperature disproportionation is difficult to control and produces titanium disulfide with substantial crystalline imperfections. Another process for preparing titanium disulfide is to react titanium tetrachloride with hydrogen sulfide to form titanium disulfide and hydrogen chloride. Titanium disulfide formed by the reaction of titanium tetrachloride and hydrogen sulfide contains substantial amounts of chlorine, i.e., up to 2 percent chlorine, which materially lowers the reversibility of the battery system due to chlorine contamination of the anode metal, e.g., lithium. Titanium disulfide has also been prepared by directly reacting titanium metal with elemental sulfur at temperatures in excess of 600.degree. C. The use of temperatures in excess of 600.degree. C. introduces a substantial number of crystal imperfections in the resulting titanium disulfide and results in a product having undesirably large average particle size and aspect ratio.